Cellular network systems called LTE (Long Term Evolution), which have been standardized by standardization organizations “3GPP (3rd Generation Partnership Project)”, (hereinafter, referred to as LTE systems) are well known.
FIG. 10 is a conceptual diagram showing an example of a configuration of an LTE system. In FIG. 10, each base station device (E-UTRAN NodeB) eNB_m provides its cell Cell_m. Adjacent cells Cell_m are located to partially overlap with each other. Each base station device eNB_m is connected to communication equipment S-GW called a serving gateway (Serving Gateway). Each terminal device (user equipment) UE is connected to a single base station device eNB_m. The communication equipment S-GW establishes a communication link with the terminal device UE through the base station device eNB_m. Herein, a communication link that the communication equipment S-GW establishes with the terminal device UE through the base station device eNB_m is called a bearer. The communication equipment S-GW serves as an anchor point (i.e. a point of executing switching between communication paths) when the terminal device UE changes its connection destination, i.e. the base station device eNB_m. The process of the terminal device UE changing its connection destination, i.e. the base station device eNB_m, is called handover (hand over) or handoff (hand off).
FIG. 11 is a conceptual diagram showing another example of a configuration of an LTE system. In FIG. 11, base station devices eNB_s are further installed in the LTE system of FIG. 10. Each base station device eNB_s is connected to the communication equipment S-GW. Each base station device eNB_s provides its cell Cell_s. The cell Cell_s has a smaller coverage than the cell Cell_m. Due to the usage of higher frequency bands and the necessity of accommodating multiple terminal devices UE in the future, engineers have studied to introduce the LTE system of FIG. 11. Hereinafter, the base station device eNB_m and the base station device eNB_s will be collectively referred to as “base station devices eNB” unless they need to be discriminated from each other.
In the 3GPP, engineers have studied an architecture (see Non-Patent Literature 1) in which, when the communication equipment S-GW establishes bearers with the terminal device UE through the base station device eNB, part of bearers is held by a base station device eNB while other bearers are established with the terminal device UE through another base station device eNB. FIGS. 12 and 13 are conceptual diagrams showing examples of the above architecture.
[Architecture A1 (See FIG. 12)]
FIG. 12 shows an architecture called “1A”. The architecture 1A is configured to establish bearers between the communication equipment S-GW and the terminal device UE through different base station devices eNB. In FIG. 12, a bearer Br1 is established between the communication equipment S-GW and the terminal device UE through a base station device eNB(a). Additionally, a bearer Br2 is established between the communication equipment S-GW and the terminal device UE through a base station device eNB(b). According to the architecture 1A, it is possible to assume normal communication paths established between the communication equipment S-GW and the base station device eNB and between the base station device eNB and the terminal device UE in units of bearers; hence, it is possible to carry out communications between the communication equipment S-GW and the terminal device UE through different communication paths for different bearers.
[Architecture 3C (See FIG. 13)]
FIG. 13 shows an architecture called “3C”. According to the architecture 3C, a single communication path is established between the communication equipment S-GW and the base station device eNB. As communication paths between the base station device eNB and the terminal device UE, part of bearers passing via the base station device eNB are divided into multiple bearers, and then part of the divided bearers reach the terminal device UE through another base station device eNB. In FIG. 13, a bearer Br3 passing via the base station device eNB(a) is established between the communication equipment S-GW and the terminal device UE. The bearer Br3 is not divided into multiple bearers. Additionally, a bearer Br4 passing via the base station device eNB(a) is established between the communication equipment S-GW and the terminal device UE. The bearer Br4 is divided into a bearer Br4(a) and a bearer Br4(b) by the base station device eNB(a). The bearer Br4(a) derived from the base station device eNB(a) directly reaches the terminal device UE. On the other hand, the bearer Br4(b) derived from the base station device eNB(a) reaches the terminal device UE through the base station device eNB(b). Transferring traffic between the base station devices eNB(a) and eNB(b) is carried out using a logical line called a line X2.
According to the aforementioned architectures 1A and 3C, the process of the terminal device UE establishing bearers with the communication equipment S-GW through a plurality of base station devices eNB is called dual connectivity. In the condition of dual connectivity, the base station device eNB holding bearers conveying control information is called a master base station (MeNB (Master E-UTRAN NodeB)) while a base station device eNB serving as a destination of transferring part of bearers is called a secondary base station device (SeNB (Secondary E-UTRAN NodeB)).